The invention relates to a circuit arrangement for switching a current as a function of a predefined switching signal. The circuit arrangement comprises a semiconductor circuit breaker for switching a current and an actuation device for the semiconductor circuit breaker. The actuation device is so designed as to receive the switching signal and to generate a control voltage at a control input of the semiconductor circuit breaker as a function of the received switching signal.
A circuit arrangement of this type is disclosed in WO 2008/032113A1. Such a circuit arrangement may be provided e.g. in a controlled converter which can be used to operate a three-phase machine.
The way in which a controlled converter functions is explained in greater detail below with reference to FIG. 1. Alternating currents I1, I2, I3, which together form a three-phase current that can be used to operate an electrical machine 18, can be generated in phase conductors 12, 14, 16 by means of a converter 10 using a direct voltage Uzk. The direct voltage Uzk may be provided e.g. between two conductor rails ZK+, ZK− of an intermediate circuit of a frequency converter. For the purpose of generating the alternating currents I1, I2, I3, the phase conductors 12, 14, 16 are connected to the conductor rails ZK+, ZK− via a halfbridge 20, 22, 24 in each case as shown in FIG. 1. The way in which the alternating currents I1, I2, I3 are generated is explained below with reference to the halfbridge 20. This explanation applies likewise to the alternating currents I2 and I3 in connection with the halfbridges 22 and 24.
The halfbridge 20 has two semiconductor circuit breakers 26, 28, each of which respectively has a transistor Tr1 or Tr2 and a diode V1 or V2 which is antiparallel connected to the respective transistor. The phase conductor 12 is connected once to the plus conductor rail ZK+ and once to the minus conductor rail ZK− via the semiconductor circuit breakers 26, 28. The transistors Tr1, Tr2 can be e.g. IGBTs (insulated gate bipolar transistors) or MOSFETs (metal oxide semiconductor field effect transistors). The semiconductor circuit breakers 26, 28 are connected to a control unit 34 via a control line 30, 32 in each case. The control unit 34 generates a clock signal 36, which is transmitted via the control line 30 to the semiconductor circuit breaker 26. The transistor Tr1 of the semiconductor circuit breaker 26 is switched alternately into conducting and a blocking state by means of the clock signal 36. The control unit 34 transmits a push-pull signal to the semiconductor circuit breaker 28 via the other control line 32, such that the transistor Tr2 of the semiconductor circuit breaker 28 is switched in counter-phase with the transistor Tr1. The alternate switching of the transistors Tr1 and Tr2 generates an alternating voltage in the phase conductor 12 and hence the alternating current I1. In order to generate the three-phase current, the control unit 34 correspondingly transmits phase-offset clock signals to circuit breakers of the other halfbridges 22 and 24 via further control lines. An alternating voltage which is generated by the electrical machine 18 can be rectified by means of the diodes of the semiconductor circuit breakers.
The clock signals which are generated by the control unit 34, e.g. the clock signal 36, are not normally present in a form that can be used to switch a semiconductor circuit breaker. Therefore an actuation circuit 40 is connected ahead of a control input 38 of the semiconductor circuit breaker 26 and generates a control voltage at the control input 38 by means of a driver circuit (not shown) as a function of the clock signal 36. In the case of a transistor, the control input 38 is the gate or base thereof. In the same way, a corresponding actuation circuit is connected ahead of the semiconductor circuit breaker 28 and corresponding actuation circuits are connected ahead of the circuit breakers of the bridges 22 and 24.
When a current is switched by means of e.g. the semiconductor circuit breaker 26, it must be considered that a voltage might be induced as a result of inductance (not shown in FIG. 1) in the circuit, depending on how fast the semiconductor circuit breaker 26 is switched from a conducting state to a blocking state. This induction voltage is then superimposed on the operating voltage, such that a resulting value of the voltage dropping over the semiconductor circuit breaker 26 may be higher than a maximal permitted value. Consequently, components of the semiconductor circuit breaker 26 may be damaged. Provision can therefore be made for a control voltage having a flattened profile, in relation to the profile of the switching signal 36, to be generated at the control input 38 by the actuation circuit 40, at least in the case of those switching edges of the switching signal 36 which are used to block the semiconductor circuit breaker 26.
In order to obtain a control voltage which has a flattened profile, the teaching of publication WO 2008/032113 A1 relates to measuring a current flowing through the power semiconductor and determining an extreme point (a maximum or a minimum) in the temporal profile of the current strength during a switching action. Until the extreme point is reached, a control voltage is varied only slowly at the control input of the power semiconductor, such that a ramp-like profile of the control voltage is produced and a conductivity of the power semiconductor only changes relatively slowly. After the extreme point is reached, the control voltage then switches abruptly to its final value.
One disadvantage of such a solution is that the switching losses in the power semiconductor are very high for the period during which the control voltage is reduced only slowly with a ramp-like profile.